Re: [Xen-devel] [PATCH v3 0/4] x86: accommodate 32-bit PV guests with SMEP/SMAP handling

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/05/16 18:02, Wei Liu wrote:
> On Thu, Mar 17, 2016 at 01:50:39AM -0600, Jan Beulich wrote:
>> As has been explained previously[1], SMAP (and with less relevance
>> also SMEP) is not compatible with 32-bit PV guests which aren't
>> aware/prepared to be run with that feature enabled. Andrew's
>> original approach either sacrificed architectural correctness for
>> making 32-bit guests work again, or disabled SMAP also for not
>> insignificant portions of hypervisor code, both by allowing to control
>> the workaround mode via command line option.
>>
>> This alternative approach disables SMEP and SMAP only while
>> running 32-bit PV guest code plus a few hypervisor instructions
>> early after entering hypervisor context from or late before exiting
>> to such guests. Those few instructions (in assembly source) are of
>> course much easier to prove not to perform undue memory
>> accesses than code paths reaching deep into C sources.
>>
>> The 4th patch really is unrelated except for not applying cleanly
>> without the earlier ones, and the potential having been noticed
>> while putting together the 2nd one.
>>
>> 1: move cached CR4 value to struct cpu_info
>> 2: suppress SMEP and SMAP while running 32-bit PV guest code
>> 3: use optimal NOPs to fill the SMEP/SMAP placeholders
>> 4: use 32-bit loads for 32-bit PV guest state reload
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Release-acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

And applied.

~Andrew

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